Implantable medical devices are often exposed to a number of mechanical tests before being used in human patients. The mechanical tests expose test samples of the medical device to a variety of forces so that the mechanical integrity of the device can be investigated. For example, medical stents are often subjected to a battery of mechanical tests that assess the behavior of the medical stent in response to bending, compression, tension, and radial pulsation. The mechanical tests may assess whether the stent can sustain the maximum expected load, or cyclical loads of long-term use, without failing. Thereby, the safety and efficacy of the stent can be identified before the device is implanted in humans.
Many known mechanical testing systems are not suited for performing the range of mechanical tests to which a stent is normally subjected. For example, one known mechanical testing system is a uni-axial test machine, which features a crosshead that oscillates within a stationary frame. A test stent is mounted to the stationary frame, and as the crosshead moves up and down, a load is applied to the stent.
One problem with the uni-axial test machine is that it applies force in only one direction. The force is applied along a line of action that is parallel to the direction of travel or plane of motion of the crosshead. Thus, the uni-axial test machine is suited for testing modes that require the application of a uni-axial force to the test stent, such as compression and tension tests, but not for other testing modes that require the application of different forces in different directions.
To adapt the uni-axial test machine for other tests, a fixture may be employed. One known fixture adapts the uni-axial test machine for bending tests. The fixture includes two supports for supporting the test sample within the stationary frame. The fixture is placed in the stationary frame, and the test sample is rested on the fixture suspended between the supports. The uni-axial machine then applies a linear force to the test sample about its mid-point, causing the test sample to bend. Because the bending force is applied to one point of the test sample, and each of the two supports exert a return force on the test sample, such a bending mode is known as “three-point bending”.
Testing modeled on three-point bending may not realistically mimic the manner in which a stent experiences bending within the body. Forces are not usually applied within the body to a single point of application. Additionally, forces are not usually applied within the body in a perpendicular direction. Because the three-point bending test applies force in an unnatural manner, the bending that the test stent experiences may be misleading. The mid-point of the stent, which is directly contacted with the load, may experience unreasonably high compression or deformation. Conversely, the end points of the stent, which are not directly contacted with the load, may experience relatively little or no bending, as the load may not be uniformly transferred along the length of the stent.
Despite these issues, testing modeled on three-point bending is often employed because the uni-axial test machine is readily available. Such a machine is standard equipment in many testing laboratories. The stent is then subjected to other testing modes separately. The separate application of other test modes does not adequately mimic the conditions in the body, where the stent may experience bending in combination with tension, compression, torsion, or radial pulsation.
From the above, a need exists for mechanical testing systems and methods that can subject a medical device such as a stent to realistic bending forces. It may be desirable if the systems and methods are suited for use with a conventional uni-axial test machine, which is available in many lab environments. If also may be desirable if the systems and methods are suited for subjecting the medical device to one or more additional testing modes simultaneously with bending.